Arsenic and phosphorus compounds which are used in doping and thin film deposition processes in the manufacture of silicon and compound semiconductor devices are extremely toxic and present handling hazards in their storage, distribution and use. For example arsine which is extensively used in doping operations for the manufacture of silicon integrated circuits and in the deposition of arsenic in chemical vapor deposition processes associated with the manufacture of gallium-arsenic devices is one of the most toxic substances known to man. It has a Lowest Concentration (LC50), wherein 50% of a sample population dies, of approximately 5 ppm translating to a Threshold Limit Value, which is the highest level allowed by law for an 8 hour workday, (TLV) of 50 ppb. Arsine is shipped and stored as a high pressure gas, which increases its potential hazard as a atmospheric contaminant. Because of this toxicity, extensive safety precautions and fail-safe equipment must be built in to processes where arsine is used. Additionally, local and national environmental laws and regulations are being enacted to further control arsine's use, shipment and storage.
In order to avoid the toxicity problems associated with arsine and phosphine, alkyl Group V organometallics have been used as sources for these corresponding elements. Tertiary butyl arsine (H.sub.2 As[t-C.sub.4 H.sub.9 ]), diethylarsine (HAs[C.sub.2 H.sub.5 ].sub.2), and trimethylarsine (As[CH.sub.3 ].sub.3) have been tested as arsine substitutes, among other alkyl arsines. These materials are somewhat less toxic than arsine. Tertiary butyl arsine has a Threshold Limit Value (TLV) equal to 80 ppb and a Lowest Concentration (LC50) equal to 70 ppm. Toxicity trends generally decrease with increased alkyl substitution. These materials will deposit arsenic in chemical vapor deposition processes, but are less desirable because they also can lead to co-deposition of carbon which is detrimental to the electrical performance of the final device.
Decomposition of Group VA organometallic and fluorinated derivatives are reported by S. J. W. Price et al, "The Pyrolysis of Trimethylarsine", Canadian Journal Chemistry, vol. 48, (1970) pp 3209-3212; and by P. B. Ayscough et al, "Kinetics of the Pyrolysis of Trimethylarsine, Tristrifluoromethylarsine and Related Compounds; Journal of the Chemical Society, (1954) pp 3381-3388.
In ion implantation doping applications, arsenic metal is sometimes used as the source. Heated to temperatures sufficient for sublimation, the vapor is directed to the implanter. However a more volatile source is usually preferred and arsine is still used in most applications.
U.S. Pat. No. 4,734,514 is directed to the use of hydrocarbon-substituted analogs of phosphine and arsine for various depositions and doping techniques.
Published European Patent Application 0 206 764 discloses techniques for doping glass to produce arseno-silicate glass. The glass is used as waveguide material.
G. R. A. Brandt, et al. in an article "Organometallic and Organometalloidal Fluorine Compounds Part V. Trifluoromethyl Compounds of Arsenic", Journal of the Chemical Society, 1952, pg. 2552-2555, reported the interaction of arsenic and trifluoroiodomethane to produce tris-trifluoromethylarsine and idobistrifluoromethylarsine.
H. J. Emeleus, et al. in an article "Organometallic and Organometalloidal Fluorine Compounds, Part VI, Trifluoromethyl Arsenicals", Journal of the Chemical Society, 1952, pg. 1552-1564, reports the reactions of various trifluoromethyl arsenicals to prepare the corresponding halides and related compounds. The article reports the decomposition of bis-trifluoromethylarsine to tris-trifluoromethylarsenic, arsenic metal and fluoroform. Tris-trifluoro-methylarsine was also subjected to ultraviolet light in a silica tube which resulted in a film of arsenic deposited in the cooler portions of the tube.
F. W. Bennett, et al. reported in an article "Organometallic and Organometalloidal Fluorine Compounds, Part VII, Trifluoromethyl Compounds of Phosphorus", Journal of the Chemical Society, 1953, pg. 1565-1571, the interaction of phosphorus and trifluoroidomethane resulted in tris-trifluoromethylphosphine.
W. R. Cullen in an article "Perfluoroalkyl Arsenicals, Part I The Preparation of Alkyl Perfluoroalkyl Arsenicals", reported in Canadian Journal of Chemistry, Volume 38, 1960, pg. 439-443, describes the reaction of tetramethyldiarsine with trifluoroidomethane to give dimethyltrifluoromethylarsine.
W. R. Cullen in an article "Perfluoroalkyl Arsenicals, Part II The Preparation and Properties of Aryl Perfluoroalkyl Arsenicals", reported in Canadian Journal of Chemistry, Volume 38, 1960, pg. 445-451, describes the reaction of iododiphenylarsine with trifluoroiodomethane to produce diphenyltrifluoromethylarsine. Thermal decomposition of diphenyltrifluoromethylarsine resulted in undisclosed black solid deposit.
In an article by M. J. Ludowise, et al. entitled "Use of Column V Alkyls in Organometallic Vapor Phase Epitaxy (OMVPE)", reported in SPIE, Volume 323, Semiconductor growth technology "1982", pg. 117 thru 124, the use of trimethylarsenic and trimethylantimony for deposition of Group III-V compound semiconductors is discussed. Specifically the organometallic vapor phase epitaxial reaction deposition of trialkyl aluminu, gallium and indiums with trialkyl and hydrides of nitrogen, phosphorus, antimony and arsenic are disclosed.
In an article by M. J. Cherng, et al. entitled "GaAs.sub.1-x Sb.sub.x Growth by OMVPE", reported in Journal of Electronic Materials, Volume 13, No. 5, 1984, pgs. 799 thru 813, the methods for organometallic vapor phase epitaxy using trimethyl gallium, antimony, and arsenic as source materials are disclosed. Much of the experimental results reported in the article were derived from using trimethylarsenic in contrast to arsine.
In an article by D. M. Speckman, et al. entitled "Alternatives to Arsine: the Atmospheric Pressure Organometallic Chemical Vapor Deposition Growth of GaAs Using Triethylarsenic", reported in Applied Physics Letter, Volume 50 (11), 16 Mar. 1987, pg. 676-678, the problem of reactant toxicity and carbon impurity contamination is highlighted in a discussion of the homoepitaxial growth of gallium arsenide layers by organometallic chemical vapor deposition. The traditional use of arsine and trimethyl gallium is contrasted with the more beneficial use of triethyl gallium. The use of triethyl gallium tends to reduce the amount of incorporated carbon into the epitaxial film.
In an article by C. H. Chen, et al. entitled "Use of Tertiary Butyl Arsine for GaAs Growth", reported in Applied Physics Letter, Volume 50 (4), 26 Jan. 1987, pg. 218-220, the substitution of tertiary butyl arsine as a new organometallic source in replacement of arsine which results in improved organometallic vapor phase epitaxial growth with diminished carbon incorporation into the gallium arsenic layer while diminishing hazards of toxicity, purity and storage in comparison to arsine was reported.
R. M. Lum, et al., in an article entitled "Use of Tertiary Butyl Arsine in the Metal Organic Chemical Vapor Deposition Growth of GaAs", reported in Applied Physics Letter, Volume 50 (5), 2 Feb. 1987, pg. 284-286, again discuss the benefits of using tertiary butyl arsine in reactions to form epitaxial films of gallium-arsenide. The results are identified as being more beneficial than using an arsenic source of trimethylarsenic.
M. J. Cherny, et al., in Journal of Crystal Growth, 77, (1986), pp. 408-417, "MOVPE Growth of GaInAsSb", describe the use of trimethyl compounds to make the name product. Similar disclosures were made at page 392-399 of the publication by G. J. Bougnot, et al. in "Growth of Ga.sub.1-x Al.sub.x Sb and Ga.sub.1-x In.sub.x Sb by Organometallic Chemical Vapor Deposition", and at page 400-407 by R. M. Biefield in "The Preparation of InAs.sub.1-x Sb.sub.x Alloys and Strained-Layer Super-lattices by MOCVD."
In an article by James H. Comfort, et al. entitled "In-situ Arsenic Doping of Epitaxial Silicon at 800.degree. C. by Plasma Enhanced Chemical Vapor Deposition" appearing in Applied Physics Letter, 51(19), 9 Nov. 1987, pages 1536-1538, a discussion is presented of the arsenic doping of epitaxial silicon films using chemical vapor deposition wherein arsine is the source of the arsenic doping.
In an article by Jacques S. Mercier entitled "Rapid Flow of Doped Glasses for VLSIC Fabrication" appearing in Solid State Technology, July 1987, pages 85-91, a report on doping of phosphosilicate glass and borophosphosilicate glass was given. The effects of flow of such glass on electronic devices is specifically identified.
In an article by R. M. Lum, et al. entitled "Investigation of Carbon Incorporation in GaAs using .sup.13 C-enriched trimethylarsenic and .sup.13 CH.sub.4 ", appearing in Journal of Electronic Materials, Vol. 17, No. 2, (1988), pp. 101-104, it was reported that increased occurrences of carbon impurity contamination of chemical vapor depositions of gallium arsenide are achieved when the less toxic trimethylarsenic is used in place of arsine.
In another article by R. M. Lum, et al. entitled "Investigation of Triethylarsenic as a Replacement for Arsine in the Metal-organic Chemical Vapor Deposition of GaAs", appearing in Applied Physics Letter, Vol. 52, (18) May 2, 1988, the use of the named arsenic compound was discussed, but problems with carbon deposition were noted.
In an article by A. Tromsom Carli, et al. entitled "Metal Organic Vapour Phase Epitaxy of GaAs: Raman Studies of Complexes Formation", reported in Revue Physical Applications, Volume 20 (1985), pgs. 569-574, the use of trimethyl arsenic in place of arsine in a metal organic vapor phase epitaxy to produce gallium-arsenic was described. The article summarizes sources of contamination in epitaxial electronic layers, wherein such contamination includes carbon, silicon and zinc.
In U.S. Pat. No. 4,721,683, tertiary alkyl phosphine and arsine compounds are disclosed to have utility for semiconductor ion implantation.
In Chapter 5 from VLSI Technology, edited by Sze, pp 169-217, (1983), diffusion applications are described in detail. Diffusion sources of phosphorus are generally implanted, but can be vapor sourced from P.sub.2 O.sub.5. Arsine diffusion sources are generally ion implanted. Diffusion was further described in B. Swaminathan, et al. "Diffusion of Arsenic In Polycrystalline Silicon", Applied Physics Letter, 40(a) May 1982, pp 795-798; and in T. Kook et al "Diffusion of Dopants in (111) Silicon During High Temperature Heat Treatment in Nitrogen", Material Research Society Symposium Proc., Vol 36, (1985) pp 83-88.
Chemical beam epitaxy is a specialized form of epitaxial growth which has been shown to be amenable to organometallic source materials. Such demonstrations are found in W. T. Tsang, "Chemical Beam Epitaxy of InP and GaAs", Applied Physics Letter, vol. 45, No. 11, 1 Dec. 1984 pp 1234-1236; W. T. Tsang, "Chemical Beam Epitaxy of InGaAs", Journal of Applied Physics, 58(3), August 1985, pp. 1415-1418, and H. Kroemer, "MBE Growth of GaAs on Si. Problems and Progress", Material Research Society Symposium Proceedings, (67), (1986), pp 3-14.
Trimethylamine alane is disclosed as having utility in molecular beam epitaxy to produce AlGaAs in a publication, "Growth of High Quality AlGaAs by Metalorganic Molecular Beam Epitaxy Using Trimethylamine Alane", by C. R. Abernathy et. al. in Applied Physics Letter, Vol. 56 (26), pp. 2654-2656, Jun. 25, 1990.
Arsine and phosphine are used extensively in organometallic vapor phase epitaxy of Group III-V compound semiconductor materials. Typically in this technique, a metal alkyl organometallic compound such as trimethyl aluminum, trimethyl indium, and/or trimethyl gallium (or other alkyl sources such as the corresponding ethyl compounds) is combined with the hydride of arsenic or phosphorus at high temperature to form single crystal (epitaxial) AlGaAs, InGaAs, InP, or GaAs, or any other mixed compound semiconductor material. Arsine is also used extensively as an n-type dopant in the epitaxial growth of silicon electronic materials, or cracked and implanted, using ion implanters, into silicon device structures. Phosphorous is used in the production of BPSG (borophosphosilicate glass) and PSG (phosphosilicate glass) glass, and similarly arsine can be used for ASG (arsenosilicate glass), to enhance reflow properties of the glass. In all these cases, arsine and phosphine are the volatile carriers of arsenic and phosphorus to the appropriate material or process.
Arsine is known to be a highly toxic material, with a Threshold Limit Value (TLV) of 0.05 ppm and a toxic limit near 5 ppm. Phosphine is not quite as toxic as arsine, but is still highly hazardous with a TLV of 5 ppm. Both are high pressure compressed gases under normal use conditions, or are supplied as high pressure mixtures in hydrogen. One of the major costs in performing Organometallic Vapor Phase Epitaxy (OMVPE), are costs associated with the installation of safety systems, including high efficiency ventilation systems, elaborate detection and alarm systems, isolated gas storage facilities, emergency purge systems and elaborate gas scrubber systems. In some areas, local regulations have gone as far as to prohibit the storage or use of arsine within municipal boundaries, or have required such elaborate safety systems as to make usage costs prohibitive. In other cases, individual company or academic safety policies have been written which have the same effect as local legislation; resultant prohibitive costs. A number of research individuals have also voluntarily refused to use arsine, due to a personal assessment of unnecessary risk. Although users of phosphine have not yet had to install such elaborate safety systems as with arsine, standard practices for phosphine usage are moving in this direction.
In silicon devices, it is often desired to dope during epitaxial growth with an n-type dopant. Typically, arsine is used during the in-situ doping step. Because of similar safety considerations as those for the GaAs semiconductor industry, silicon device manufacturers have been forced to move away from in-situ arsenic doping to post-epitaxial arsenic ion implantation. Arsine is electrically ionized under high voltage conditions forming ions of As, As2, As3, etc., and implanted through an ion accelerator into the silicon surface. An alternative to arsine in the ion implantation step is elemental arsenic (or elemental phosphorus for phosphorus implantation). However, the preferred materials remain arsine and phosphine, due to process logistics.
Diffusion of Group VA elements into silicon is used to form bases, emitters, resistors, source and drain regions, and doping of polycrystalline silicon. The Group VA element can be introduced into the silicon from diffusion of a chemical source in a vapor form at high temperatures, diffusion from a doped-oxide source, or diffusion and annealing from an implanted layer.
Researchers in the electronics field have examined using possible substitutes for arsine and phosphine. However, some compromises had to be made especially in attempts to effectively grow compound semiconductor material. Experimental focus has been on the per-alkyl and alkyl hydrides of the Group V elements, e.g., trimethyl arsenic, dimethyl arsine, diethyl arsine, t-butyl arsine, t-butyl phosphine, and di-n-butyl phos- phine, among others. Although the toxicities of these materials would be somewhat less than AsH.sub.3 or PH.sub.3, the hazard levels are seen to be much lower because they are not compressed gases. For instance, tert-butyl arsine (TBAs) has an LC50 of 70 ppm in rats via inhalation, corresponding to an OSHA TLV/TWA ceiling limit of 0.5 mg/m.sup.3, or roughly 80 ppb. This toxicity limit is almost on par with arsine itself, which has a TLV/TWA of 50 ppb. Because the material is a liquid with a boiling point of 69.degree. C., the hazard level is seen to be much lower.
Carbon incorporation into grown films of GaAs or other compound semiconductor materials is a serious problem with these substitutes. The reasons for incorporation of carbon into an epitaxial compound semiconductor layer are twofold. Metal Group V carbon bonding is fairly stable to thermal decomposition, therefore requiring fairly high growth temperatures. The strength of this metal-carbon bond, even at these high temperatures, allows a fairly high surface concentration of methyl radicals to build up, which can be incorporated during epitaxial growth. Additionally, the methyl radicals can form stable surface complexes with both Group III and Group VA surface elements. Methyl radicals can eliminate hydrogen easily at these process temperatures, as this bond is the weak bond in the system for the Group III arsenides and/or phosphides, and incorporate the carbon into the compound semiconductor crystal lattice. Carbon preferentially incorporates into the Group VA element lattice site. Additionally, methyl radicals are adsorbed and desorbed in the boundary layer, leading to further migration and incorporation of carbon. Lowering the system pressure during growth drives the adsorption/desorption equilibrium towards desorption, lowering the amount of incorporated carbon somewhat. Trimethyl arsine has been documented as giving fairly severely carbon doped GaAs epitaxial films. Tert-butyl arsine has been used in attempts to minimize carbon incorporation, due to the stability of the tert-butyl organic fragment produced during pyrolysis. The t-butyl fragment is too sterically hindered to fit into the GaAs crystal lattice, giving less severely doped films. Additionally, the strong carbon-carbon bonds must be broken to incorporate carbon from this fragment. However, it is standard practice in the MOCVD growth of InSb to use trimethyl antimony as the Sb source, with few problems related to intrinsic carbon incorporation.
In the area of phosphorus doping for BPSG (borophosphosilicate glass), there are some commercially acceptable alternatives to phosphine, such as phosphorus oxychloride, or trimethyl phosphite, known as POCl and TMP, respectively, in the electronics industry. However, there are no arsenic sources, which could substitute for arsine in ASG (arseno-silicate glass) production that have found industry favor or utility.
U.S. Pat. No. 4,904,616 discloses arsenic, antimony and phosphorus derivatives having fluorocarbon character, which lessen the drawbacks of using such atoms in electronic materials fabrication.
Amides of arsenic, antimony and phosphorus are discussed in the treatise, METAL AND METALLOID AMIDES by Lappert et al., Ellis Horwood Ltd., 1980, Chapters 6 and 7, pages 382-464, wherein syntheses and various compositions are set forth.
The above-identified prior art sets forth the state of reactive deposition of Group III and Group V compounds for production of epitaxial layers and various doping applications of pre-existing layers and glasses. The prior art recognizes the drawbacks in the use of arsenic, phosphorus antimony and other metal complexes as reactant sources which have difficult transportation and storage, problematic purity maintenance, acute toxicity concerns, as well as contribution to impurity formation in the end product which are derived from such reactant sources. These problems of the prior art have been overcome by the improved processes of the present invention set forth below.